The invention relates to a sewing machine, and more particularly to a bobbin thread tensioning device for a sewing machine having a loop taker, and a bobbin carrier for a lower thread.
According to the prior art, the adjustment of the bobbin thread tension has been made by fastening or unfastening a screw which secures a thread tensioning leaf spring to a bobbin or a bobbin carrier. Such an operation is possible only when the sewing machine is stopped to open the needle plate and take out the bobbin or bobbin carrier carrying the bobbin thread tensioning leaf spring. Such an adjustment is burdensome, and moreover it is unefficient to interrupt the machine running operation. Further in a case that the rotation torque of the bobbin is adjusted for adjusting the tension of the lower thread, the difference in the tension is brought about, since the difference in the thread remaining amount is due to difference in radius of the rotation of supplying the thread though the rotation torque is made constant.